The Basic Input/Output System (BIOS) is the first code run by a computer when the computer is powered on. The primary function of the BIOS is to load and start an operating system. When a computer starts up, the BIOS initializes system devices and locates those devices from which an operating system can be loaded. The BIOS then loads the operating system from the device. This process is known as booting, booting up, or bootstrapping.
The BIOS in a virtual machine system can boot up guests hosted by the system according to a default boot order. The boot order recognized by the BIOS can be a list of device types; e.g., disk, CD-ROM, network, floppy. However, a virtual machine system can have multiple devices of the same device type (e.g., multiple disks). Thus, the BIOS needs to check each individual device of the same device type in order to find a device that stores boot software (e.g., an operating system and other software used for booting) from which a guest can be booted. When different boot software is stored in multiple devices of the same device type, the BIOS may boot a guest from one device while the user intends it to boot from another device of the same device type. A system administrator cannot control the order of devices that the BIOS should follow when searching for boot software in a system that has multiple devices of the same device type.